Serial Attached SCSI (SAS) Expanders facilitate communication between large numbers of SAS devices. Expanders contain two or more external expander-ports. Each expander device contains at least one SAS Management Protocol target port for management and may contain SAS devices itself. For example, an expander may include a Serial SCSI Protocol target port for access to a peripheral device. An expander is not necessary to interface a SAS initiator and target but allows a single initiator to communicate with more SAS/SATA targets. A useful analogy: one can regard an expander as akin to a network switch in a network which allows multiple systems to be connected using a single switch port. Expanders exist to allow more complex interconnect topologies. Expanders assist in link-switching (as opposed to packet-switching) end-devices (initiators or targets). They may locate an end-device either directly (when the end-device is connected to it), via a routing table (a mapping of end-device IDs and the expander the link should be switched to downstream to route towards that ID), or when those methods fail, via subtractive routing: the link is routed to a single expander connected to a subtractive routing port. If there is no expander connected to a subtractive port, the end-device cannot be reached.
The current SAS switch uses a system of dedicated inter expander links (IELs) to connect two expanders together into a system that provides port redundancy and improved performance. If this type of system were to be used to connect four expanders, the interconnects would be spread out between more expanders, resulting in fewer IELs per expander pair resulting in lower bandwidth between expanders. As the number of interconnected expanders increases, the problems get worse.
Consequently, it would be advantageous if an apparatus existed that is suitable for interconnecting four or more expanders together while maintaining bandwidth between the expanders.